but when i launch any customer's cpanel, i get this error:
"Sorry for the inconvenience!
The filesystem mounted at / on this server is running out of disk space. cPanel operation has been temporarily suspended to prevent something bad from happening. Please ask your system admin to remove any files not in use on that partition."

for completeness, here's the fix from that thread, which did indeed fix the problem:

We believe this error is due to a bug in FreeBDS's statfs system call. The
latest cPanel builds have an option to disable this system call and use output
from df instead. Running the following commands should correct this error: